Medical applications of light have been widely studied. When light is to be delivered to a site within a patient's body, such as a blood vessel or other body passage, an optical fiber is typically used for transmission of the light. Optical fibers have small diameters, are relatively flexible and can deliver reasonably high energy levels. An example of the application of light is laser balloon angioplasty, which involves the simultaneous application of laser energy and pressure to a plaque-narrowed region of an artery, as described in U.S. Pat. No. 4,799,479 issued Jan. 24, 1989 to Spears. Optical fiber devices usually have an element at the distal tip for controlling the output pattern of the light. Various optical fiber tip configurations have been described, including a lens for expanding or focusing the light, a heating element for converting the light energy to thermal energy and a diffusing tip for directing the light outwardly in a generally cylindrical pattern. Optical fiber devices may be used for a variety of applications, including but not limited to tissue vaporization, tissue coagulation, photoactivation of drugs and photopolymerization.
A diffusing tip is typically used when a generally uniform light output pattern is required over a prescribed length of a body passage. Several prior art techniques have been disclosed for directing light outwardly from the diffusing tip of an optical fiber. A tapered optical fiber tip surrounded with a diffusing medium for light radiation treatment of tumors is disclosed in UK Patent Application No. 2,154,761, published Sep. 11, 1985. A tapered optical fiber diffusing tip is also disclosed in U.S. Pat. No. 4,878,492 issued Nov. 7, 1989 to Sinofsky et al. An optical fiber surrounded with a scattering medium for producing a cylindrical pattern of light at the tip of an optical fiber is disclosed in U.S. Pat. No. 4,660,925 issued Apr. 28, 1987 to McCaughan, Jr. A technique for roughening the surface of an optical fiber tip to cause wide angle radiation of light is disclosed by H. Fujii et al. in "Light Scattering Properties of a Rough-Ended Optical Fiber," Optics and Laser Technology, February 1984, pages 40-44.
Optical fiber devices for medical applications have typically utilized glass or silica optical fibers. U.S. Pat. No. 4,995,691 issued Feb. 26, 1991 and U.S. Pat. No. 5,190,538 issued Mar. 2, 1993 suggest that polymeric optical fibers are suitable for medical applications.
The aforementioned U.S. Pat. No. 4,799,479 discloses a glass optical fiber that extends through a catheter and terminates in a light-disseminating tip located within an inflatable balloon. Spears teaches that the light-disseminating tip can be provided by removing the cladding from the fiber tip and roughening the fiber core surface. Optical fibers with roughened distal ends are also disclosed in U.S. Pat. No. 5,401,270 issued Mar. 28, 1995 to Muller et al. and PCT Publication No. WO91/06251, published May 16, 1991. An optical fiber diffusion tip wherein the cladding has a thickness selected to transmit a portion of the light radiation carried through the optical fiber is disclosed in U.S. Pat. No. 5,042,980 issued Aug. 27, 1991 to Baker et al. An optical fiber device wherein openings are formed in the cladding to expose the core without physical alteration of the core is disclosed in U.S. Pat. No. 5,248,311 issued Sep. 28, 1993 to Black et al.
All known prior art optical fiber diffusing tips have had one or more disadvantages. Diffusion tips wherein a glass optical fiber has a roughened core surface have a tendency to break, because the fiber is weakened in the roughened area. Glass is known to have high notch sensitivity, and thus a tendency to break at the site of a surface defect, because of the low energy of propagation of a crack. Plastics, even relatively brittle plastics such as polystyrene or polymethylmethacrylate, in contrast, have a much higher crack propagation energy, and are thus much tougher than glass. Breakage of a diffusion tip within a patient's body may have extremely serious adverse consequences. Other diffusion tips, such as those in which a scattering medium is enclosed in a transparent tube, have a larger diameter than the optical fiber and may not be usable in certain applications which require extremely small diameters. Many prior art diffusion tips involve fabrication techniques which are complex and expensive. Prior art diffusion tips have frequently exhibited non-uniform radiation patterns and/or radiation patterns which were not repeatable from unit to unit.